The Institute of Electronic and Electrical Engineers (IEEE) Standard 802.11-2016 is a reference related to this disclosure of which the entire contents are incorporated herein by reference. The IEEE 802.11 Standard is commonly referred to as “Wi-Fi”. A Wi-Fi network generally includes of an access point (AP) and a number of stations (STA).
The Standard describes the medium access scheme used in Wi-Fi. The basic medium access scheme used in Wi-Fi is CSMA/CA (carrier sense multiple access with collision avoidance). CSMA/CA reduces the probability of collisions between multiple STAs accessing the radio medium. When the medium becomes idle following a busy medium situation, the highest probability of a collision exists. This is because multiple STAs may be waiting for the medium to become idle again. In this situation the random back off procedure as defined by the CSMA/CS protocol is used to resolve the medium contention.
Note that CS, carrier sense, is the function that determines if the medium is busy or idle. The virtual CS mechanism is achieved by distributing reservation information announcing the impending use of the medium. The Duration/ID field in individually addressed frames is used to distribute the medium reservation information and this field gives, for example, the time that the medium is reserved to the end of the acknowledgment (ACK) frame sent in response to the initial frame. Another means of distribution of this medium reservation information is the exchange of RTS (ready to send) and CTS (clear to send) frames prior to a data frame, using the Duration/ID field to indicate the total medium reservation required to complete the data packet and ACK exchange following the RTS, CTS exchange. A further way to distribute this medium reservation is by the transmission of CTS frames alone. In the latter case, the medium reservation time can be set to cover an extended period. The network allocation vector (NAV) is an indicator, maintained in each station and access point, of time periods when transmission onto the wireless medium may not be initiated and this value is set by the virtual CS mechanism and the received duration values in the Duration/ID field.
There is also a physical CS mechanism that indicates that the medium is busy if any energy above a set, i.e., predetermined, level is detected at the antenna connector of the Wi-Fi device. There are differing details of the physical CS for the individual PHY (physical) specifications in the Standard. Basically, if the medium is busy, as indicated by the CS function, then a Wi-Fi device shall not transmit. The back off procedure is used before a packet is transmitted where a random delay is used after the packet is presented for transmission and the packet is actually transmitted.
FIG. 1 is a schematic diagram depicting a Wi-Fi airborne station 10 (also referred to as STA 10) that is receiving transmissions from a multitude of Wi-Fi access points and stations, 120, 130 and 140, within a coverage area 110 which is compatible with the antenna coverage of the airborne station 10. In the example depicted in FIG. 1, the airborne station 10 is attempting to communicate with one particular ground based access point 120, which in this example is an access point (AP). Within the coverage area 110 at the same time that ground based AP 120 is transmitting it is extremely likely that a number of other devices, 140, are also transmitting. Therefore at the airborne station 10 a number of unwanted transmissions 160 are being simultaneously received in addition to the wanted transmission 150. This multiple reception has two major consequences. First, the wanted transmission 150 cannot be successfully decoded at airborne station 10 due to the interference from the unwanted transmissions 160, and second, the airborne station 10 will be prevented from transmitting due to the CS determining that the medium is effectively always busy. The more networks that are in the coverage area 110, the worse the situation. If there are only a few networks, or the traffic is light, then it is possible that the CS function may periodically indicate an idle medium. A problem is when the traffic is such that the interference prevents any ground based transmissions from ground based AP 120 to airborne station 10 to be successfully received and that the CS function indicates an effectively permanent busy medium.
FIG. 2 is a timing diagram that further describes by example why airborne station 10 cannot transmit if there are a number of active networks in the coverage area 110. FIG. 2 depicts a number of ground based networks, networks 1 to 8, 201 to 208 respectively, together with examples of packets, 210, 220, 221, 222, 230, 240, 250 251, 260, 261, 262, 270. 271, 280, 281 and 282 that the networks are actively transmitting. As the networks 1 to 8, in this example, are ground based then it is assumed that they are not overlapping and hence may transmit simultaneously whereas, as airborne station 10 is airborne it does receive all of the transmissions. In this example, at time T1 295, transmission 220, from network 2 202, transmission 230, from network 3 203 and transmission 260 from network 6 206 are all being received at airborne station 10. Similarly at time T2 296, packets 210, 221, 250 261 270 and 280, from networks 1 (201), 2 (202), 5 (205), 6 (206) 7 (207) and 8 (208) respectively are all being received at airborne station 10. Similarly, in this example, at times T3 297, T4 298 and T5 299 there are multiple simultaneous transmissions and furthermore, it can be readily observed that at no time is the medium idle as seen by the airborne station 10. As a result, the CS will indicate medium busy continuously 290 at the airborne station 10. Hence airborne station 10 is unable to cause a transmission to the Wi-Fi ground based AP 120.
FIG. 3 depicts a method that may be used to enable airborne station 10 to send a transmission to ground based AP 120. This method is commonly used in ground based networks where there may be overlapping networks. The networks 1 to 8, 201 to 208 and the traffic in those networks is the same as previously shown in FIG. 2. At time t1, 311, the medium busy indication 209 at airborne station 10 is forced low 305 by a control in the airborne station 10, overriding the CS mechanism. Airborne station 10 then transmits a CTS packet 320 at time t2 312. The CTS packet contains a duration field value that is intended to reserve the medium for a time that covers the transmission of the subsequent data or management packet 330 from airborne station 10 plus the resultant ACK 340 expected from ground based AP 120. In this example, the CTS packet 320 will cause packets 210, 221, 240, and 261 to be delayed to time t6 316 as each of those networks receives the CTS and obeys the virtual NAV set by the CTS duration. The data or management packet 330 (hereafter sometimes referred to as data packet or management packet 330) from airborne station 10 at time t3, 313, also contains a duration field value that is intended to reserve the medium until the end of the expected ACK 340 that is sent after the data packet 330 is correctly received. In this example, this duration value will additionally cause packets 240 and 241 to be delayed. Ground based AP 120 receives the data or management packet 335 and, assuming it is correctly decoded, ground based AP 120 transmits the ACK 340 at time t5, 315. In this example, at the same time that the ACK 340 is transmitted by ground based AP 120, packet 251 is also being transmitted in network 5, 205, because packet 250 prevented both the CTS packet 320 and the data packet 330 from being received by network 5, 205, and hence the duration field value was not decoded and the NAV not set. Similarly packets 271 and 282 are also being transmitted because packets 270 and 280 prevented the reception of the CTS packet 320 and data or management packet 330 from being correctly received in networks 7 (207) and 8 (208) respectively. Hence, the ACK 340, transmitted by ground based AP 120 at time t5, 315 is highly likely to be blocked at airborne station 10 and as a result, airborne station 10 will interpret that the data or management packet 330 failed.
FIG. 4 depicts an example of ground based AP 120 attempting to transmit a packet 410 to the airborne station 10. Packet 410 may be a data or a management packet. For simplicity in the following explanation it is assumed that it is a data packet. The networks 1 to 8, 201 to 208 and the traffic in those networks is the same as previously shown in FIG. 2. As ground based AP 120 is a ground based device, it is assumed that it does not receive packets from the other networks, 1 to 8, 201 to 208. Therefore for ground based AP 120 the medium is idle and it may transmit. However, at the time that ground based AP 120 transmits the data packet 410 to airborne station 10, other ground based networks are already transmitting, namely packets 221, 250, 270 and 280. The likelihood is therefore that airborne 10 will not receive or be able to decode packet 410. Ground based AP 120 therefore will not receive an ACK for packet 410 and will retry the packet 411 after a random delay. At this time interference from packets 240, 261, 270 and 281 are also in the process of transmission and again the likelihood is that airborne station 10 will not receive or be able to decode packet 411. A subsequent retry, 412 after another random delay will also likely fail due to packets 251, 262, and 282. In practice, the possibility of a successful transmission from ground based AP 120 to airborne station 10, when ground based AP 120 is contained in a coverage area 110 where there are a multitude of other Wi-Fi networks, is very low.